1. Field of the Invention
The subject matter of this invention is related generally to motor controllers and is related more specifically to microprocessor-controlled motor controllers.
2. Description of the Prior Art
Motor controllers or motor starters as they are sometimes called are well known in the art. Generally, they comprises a switch or ganged switches which are operable to open or close to provide or interrupt current to the stator windings, for example, of an electric motor. The opening and closing process is controlled by appropriately arranged relay coils and relay contacts in what is well known in the prior art as a "relay ladder" arranged in a logical order to properly sequence the starting and/or stopping of the motor.
With the advent of the microprocessor it was found that part, if not all of the relay logic arrangement, could be replaced by a properly programmed microprocessor. Such arrangements are shown and described in a paper entitled "A Quantative Analysis of Grouped Single-Phase Induction Motors" published on page 125 of the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 2, March/April 1981 by J. R. Dunki-Jacobs and Robert H. Kerr; a paper entitled "Thermal Tracking--A Rational Approach to Motor Protection" by D. R. Boothman, E. C. Elgar, R. H. Rehder and R. J. Wooddall identified as IEEE Transactions Paper 274029-5 recommended for presentation at the IEEE PES Winter Meeting, New York, N.Y., Jan. 27-Feb. 1, 1974; a paper entitled "Microprocessor-Based Universal Motor Protection System" appearing in the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 1, January/February 1981 by E. B. Turner and H. Michael Willey and in a descriptive bulletin (41-560(E)) entitled MOTOGARD.TM. for motor protection by the Westinghouse Canada Switchgear and Control Division issued January 1980. A reading of all of the above will show that certain factors are of importance when discussing motor starters or motor controllers.
To insure the accurate and safe operation of motors and apparatus controlled by the microprocessor-based motor controller, it is important, for the sake of both safety and convenience of operation, that commands be accurately received from input switches and the like. Often optically coupled input devices are used in a portion of the microprocessor. However, it is recognized that either the optical device or the input switch may fail. This is particularly important if the switch fails in the closed mode or state. Generally the simplest way of determining the mode of an input switch is to read a logic level from an input device connected thereto. One reading is generally used for the decision-making process. However, errors may arise in this simple process because of contact bounce in the switch or because of noise pulses or because the switch has failed in the closed state. Therefore, it would be advantageous if apparatus could be found which could determine if the input switch-state information were reliable.